This invention relates to an apparatus for controlling an adjustable-speed electric motor.
Electric power converters or inverters have been employed for the application of adjustable-speed drives using alternating current motors. A typical converter includes a direct current (DC) rectifier for rectifying three-phase AC input voltage and for supplying the resulting direct current (DC) bus potential to an inverter. The inverter comprises a plurality of pairs of series-connected switching elements to generate an adjustable frequency output. In many applications, such a frequency adjustment is effected through a control circuit which employs a pulse width modulation (PWM) control technique in producing variable frequency gating pulses to periodically switch the respective switching elements so as to operate the motor at a variable speed. The motor can be propelled (motoring mode) or retarded (braking mode) as desired by appropriately varying the frequency and the amplitude of the excitation that the inverter applies to the motor.
The actual motor speed is sensed and compared with a commanded motor speed. An speed error signal, which depends on the difference between the actual and desired values of motor speed, is derived and applied to a proportional-plus-integral control circuit which converts it into a torque command signal. The control circuit responds to the torque command signal by controlling the operation of the inverter so as to vary, as a function of the torque command signal, the amplitude of the voltages supplied from the inverter to the motor.
One problem occurs where the motor is coupled to drive a mechanical load having elastic elements. For example, an elevator has an elevator rope, springs, rubber cushions and other elastic elements for providing improved passenger's comfort and vibration insulation. In such a case, the control circuit tends to resonate with the mechanical load. This tendency neutralize any attempt to increase the response time of the control circuit with respect to commanded motor speed changes.